The present invention relates to an improved method for preparing abrasion resistant crystals of sodium perborate tetrahydrate, and, more particularly, to a method for preparing such abrasion resistant crystals from an aqueous slurry of sodium perborate tetrahydrate using vacuum crystallization while continuously removing small nuclei, i.e., generally less than 20 microns, from the crystallization medium.
The preparation of sodium perborate tetrahydrate by a method to which this invention is particularly applicable is described in U.S. Pat. No. 2,828,183. Hydrogen peroxide is reacted with sodium metaborate in an aqueous slurry of sodium perborate tetrahydrate at a temperature of at least about 20.degree. C., but preferably at a temperature in the range of from 25.degree. to 40.degree. C. A sodium tetraborate concentration of at least 8 grams per liter is maintained in the liquid phase crystallization medium. The hydrogen peroxide and sodium metaborate feed rates are proportioned so as to avoid an excess of hydrogen peroxide in the liquid phase. Sodium perborate tetrahydrate is crystallized from the liquid phase, and the sodium perborate tetrahydrate crystals which form can then be separated from the crystallization medium by filtration or centrifugation and then dried to remove adhering moisture. While the crystals which are obtained are resistant to caking at elevated temperatures and exhibit generally good pouring characteristics, they are not resistant to abrasion, i.e., dust and fines are likely to result during handling and subsequent processing.
The present invention provides a method for preparing sodium perborate tetrahydrate crystals which have the desired abrasion resistance as well as good pouring and anti-caking characteristics.